gspawn: Declare environ
gspawn: Declare environ

Function do_posix_spawn uses environ, but gspawn.c doesn't declare it.
Since there is no system header declaring this global variable, this
causes compilation error on FreeBSD.

Code added in this commit is copied from genviron.c.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib